David Cameron has just made his worst mistake. He will bitterly regret the day he encouraged Boris Johnson to stand as London's mayor. What does it say about the desperate state of the Conservatives that they will put up a clown to run a great global city? London is the nation's powerhouse, and a city of daunting complexity. Tories running top City firms and Conservative boroughs won't find the Boris Johnson candidacy charmingly funny. Some may or may not agree with his rightwing views, but they will wince at serious London politics treated by the Tory leadership as a celebrity Eton wall game.

This will blow back on Cameron dangerously. No doubt the Boris bandwagon will be good circus entertainment, and his japes may be endlessly forgiven with one of his rumpled "Cripes!" apologies. But everything foppish, buffoonish and essentially unserious about his raffish progress through London will mirror exactly what people already think about Cameron and Osborne's Etonocracy. Everything they are trying to shake off will be writ large as Boris represents the Cameroons. They are struggling for gravitas but Boris will strip it away from them.

Of course if a monkey can be elected mayor of Hartlepool, Boris Johnson might be elected mayor of London. Jester, toff, self-absorbed sociopath and serial liar, the man could still win. Even Conrad Black called him "a duplicitous scoundrel", and he should know. But it's truly alarming that he who has never run anything except his own image could be in charge of this mighty financial centre - and some of the poorest, neediest boroughs in the country.

It would be good for London to have a serious contest: a third-term shoo-in for Ken Livingstone would be a miserable election. But it would be a disaster for London if a charming fool, with no interest in ordinary Londoners' lives, were to win it as another feather in his celebrity cap. If London is competing with New York, how does Boris shape up against Bloomberg in the big league?

The danger is that politics is so despised and politicians are so loathed that anyone who manages to seem "not one of them" starts ahead of serious contenders. It's why women candidates start with a natural advantage, according to pollsters. Hartlepool's H'Angus, after all, wasn't even a person, let alone a politician. Ladbrokes yesterday made Johnson's odds just a shade longer than Livingstone's.

But with humour and wit in short supply in politics, a little goes a long way and Johnson has a lot of both. So does Livingstone. Johnson would have the old "throw the bastards out" insurgency advantage, but Livingstone has earned respect with the bravery and skill of his congestion charge, his London bus revolution and his imposition of 50% affordable housing on every development.

Johnson's best asset is the devoted support of London's only proper newspaper. The Evening Standard - same stable as the Daily Mail - detests Livingstone: no surprise they gave Johnson front-page and leader-column coverage, with an article by himself (all about himself, not much policy) and lavish praise from the rightwing columnist Andrew Gilligan: "Boris has come to save our great city from Ken's ghastly empire of bureaucrats, bendy buses and earnest Cuban festivals." The Standard's never-ending campaign against Livingstone led to a famous fracas when he likened one of its reporters to a concentration camp guard: the reporter was Jewish.

But when it comes to gaffes, it'll be a no-contest win for Johnson who can't resist the temptation to be (charmingly) offensive in every column he writes and every lucrative speech he makes. (He earned more than £400,000 last year in journalism and after-dinner speaking on top of his MP's salary.) Can he stop himself making jokes about poodle-eating South Koreans and Papuan "cannibals"? No, but that's part of his well-honed USP as "not really a politician at all". Don't be fooled. Despite that designer shambolic demeanour, Boris is not called blond ambition for nothing, with a gargantuan appetite for everything: fame, women, money, praise - and power. There's nothing wrong with ambition; the question is: what is it for?

Underneath the whimsy and the Greyfriars pastiche prep school banter, he is a deeper and more passionately romantic believer in 19th-century Conservativism than most of his frontbench companions. He is not an embarrassed Etonian, but a Bullingdon Club believer. Perhaps because he was not born to great wealth, unlike Cameron and Osborne, he revels in everything elite - intellectual, social or monied.

Jokes make outrageous views acceptable, but the general tenor of Borisisms reveals his political cast of mind - the endless mock cockney attacks on "elf'n'safety", on children's car seats or, notoriously, Liverpudlians wallowing in their victim status. He hints at utter contempt for the NHS, with USSR comparisons. Though liberal on matters of sex (what else could he be?) and drugs ("I'm instinctively inclined to liberalise"), his politics are right off the Cameron scale. Here he is on education: "I am in favour of selection ... So is every member of the British ruling classes"; and on universities: "I believe passionately in academic inequality."

Just before the grammar school row he complained: "We have taken away the old ladder of social mobility, the academic selection that used to form a way out for the bright children of poor families." How will London parents react to the tone of this? "Masters of the Universe" should "endow new schools for improving the education of our feral children to reduce the risk of being despoiled of their squillions by a hoodie". As a rabid Europhobe, how would that play with the Olympics or the Tour de France?

What about Boris the sociopath? Apart from being caught often lying to all and sundry - he was fired from the Times for making up a quote - how has he survived the Darius Guppy scandal when he was recorded agreeing to find a journalist's contact details so old Etonian friend Guppy could have the man beaten up? How badly? Guppy suggested just a few cracked ribs. Later when Guppy was jailed for a £1.8m insurance fraud, Boris explained his role with: "Oh poor old Darry was in a bit of a hole. He was being hounded." Can Cameron really get through nearly a year's mayoral campaign by just laughing and saying, as he does, "Boris is Boris"? If he were to win, Cameron would be in a worse hole still.